1. Field of the Invention
The invention relates generally to semiconductor wafer processing systems. More specifically, the invention relates to a method and apparatus for etching dielectric films.
2. Description of the Background Art
Semiconductor wafer processing involves processes having multiple steps including deposition steps and etch steps. Many of these processes use a plasma to process the wafer. In a typical etch step, for example a plasma, generated in a process chamber, produces reactive ions, free radicals or both. These reactive species remove (etch) material from the surface of the semiconductor wafer. In the prior art etching systems, the plasma is generated from a process gas introduced to the chamber through the chamber wall by a plurality of (typically four) gas nozzles symmetrically distributed about a pedestal that supports the workpiece in the process chamber. Approximately equal amounts of gas are delivered through each of the four nozzles. A pump, connected to the chamber by a pump port located on one side of the chamber, regulates the pressure in the chamber by continuously exhausting gases. Unfortunately, byproducts of the etch process tend to collect in the vicinity of the pump port. Consequently, there are fewer reactive species near the pump port and more in other parts of the chamber. This skewing of the distribution of reactive species and byproducts in the plasma causes a non-symmetric etching of the workpiece.
In a prior art etching system (such as a metal etch DPS chamber manufactured by Applied Materials, Inc. of Santa Clara, Calif.), if symmetric gas flow is utilized (i.e. process gas flows equally through all four nozzles), as in FIGS. 3a and 3b, the etch contour maps tend to tilt toward the pump, i.e., the etch contour is not symmetric about the center of the wafer. Specifically, FIG. 3a illustrates an etch contour map for an oxide wafer and FIG. 3b illustrates an etch contour map for a BCB wafer.
Thus, there is a need in the art for a method and apparatus for improving the uniformity of the distribution of reactive ions in a plasma process to improve the symmetry of wafer processing.
The disadvantages heretofore associated with the prior art are overcome by a method and apparatus for processing a workpiece in a chamber by providing an asymmetric flow of process gas and processing the workpiece with the process gas. The asymmetric flow counteracts a non-uniform distribution of reactive species and process byproducts in the chamber. The asymmetric flow can be accomplished by opening one or more gas nozzles located proximate the pump port and blocking one or more other gas nozzles. Consequently, the process gas flows primarily through the gas nozzles located proximate the pump port. As the process gas and process byproducts produced in the chamber are exhausted through the pump port, the process gas is replenished from the nozzles near the pump. Additional control of the non-uniformity can be achieved by providing process gas through a showerhead attached to a lid of the chamber. The method of the present invention improves the uniformity of, for example, an etch process such as a dielectric etch.
The invention may be embodied in an apparatus comprising a processing chamber having a plurality of asymmetrically distributed gas nozzles communicating through a wall of the chamber located proximate a pump port.
This invention fulfills the need for a method and apparatus that improves the symmetry of plasma processes such as dielectric etch.